The proposed research involves detailed ultrastructural and biochemical investigations into the development of microvilli. The specific aims of this project are to reexamine in detail the two existing models for microvillus growth. A comparative approach will be used to examine developing intestinal microvilli from chicken embryos, amphibian tadpoles and sea urchin larvae. Work will also be conducted on developing epididymal stereocilia from embryonic and newborn mice. Through a comparative study the aim is to compile new information that will complement existing literature leading to a general model for microvillus development. The research methods to be employed include light microscopy and high resolution scanning and transmission electron microscopy of thin-sectioned and freeze-fractured material. Attention will be focused on the assembly of core filaments and their relationship to the plasma membrane and the forming terminal web. Efforts will be made to localize polymerized actin during stages of microvillus development by labeling filamentous actin in mildly extracted epithelia with myosin subfragment-1 or tropomyosin. Biochemical fractionation of intestinal epithelial cells before and after microvillus formation will be done to define the pool size of monomeric actin which should be relatively large prior to microvillus growth.